Such steel structures are widely known in the art. Recent prior art documents have disclosed a tendency towards steel structures where the steel filaments present one or another type of waviness, i.e. where, in addition to the plastic deformation as a consequence of the possible twisting of the steel filaments, the steel filaments have another plastic deformation. This additional and other plastic deformation is conveniently a consequence of a preforming operation, and results in a wavy pattern on the steel filament.
In this way U.S. Pat. No. 5,020,312 (Kokoku--priority 1989) and U.S. Pat. No. 5,111,649 (Kokoku) disclose steel cord structures consisting of three to five steel filaments. At least one steel filament is provided with a so-called `crimp`: this is a zigzagged form with relatively sharp angles, the sharpness depending upon the formation tools. The crimp is a planar wave form and is formed by means of two toothed wheels. The holes created at the level of the angles promote penetration of elastomer into the steel cord structure.
Another wave form has been disclosed in EP-A-0 462 716 (Tokusen--priority 1990). According to this document, the steel cords have three to twenty-seven steel filaments, 25% to 67% of which have a particular helix or helicoidal form. The plastical helix deformation is carried out by means of rotating preforming pins. The purpose is to promote penetration of the elastomer into the steel cord structure without increasing the so-called part load elongation (PLE, for definition see below). These steel cords are marketed under the name SPACY.RTM. cord. An important drawback of this cord is that its manufacture is energy-consuming or inefficient or both. Indeed, if the pitch of the helix is taken smaller than the twist pitch, then the rotation speed of the preforming pins must be more than twice as high as the rotation speed of a down-stream double-twister.
Still another wave form has been disclosed in WO-A-95/16816 (Bekaert--priority 1993). According to this document, the steel structure comprises steel filaments and at least one steel filament has been polygonally preformed. This is a spatial wave form and is the result of a preforming device with varying radii of curvature. The steel structures are marketed under the name BETRU.RTM..